An order of the Reptilia, subclass Anapsida, including the turtles, terrapins, and tortoises. This order is also known as the Testudines. The group first appeared in the Triassic, and its representatives are among the commonest fossils from that time on. Members of the order are most frequently found in fresh-water streams, lakes, and ponds or in marshy areas. However, a number of strictly terrestrial species are known, and several are marine. Turtles occur on all the major continents and continental islands in tropic and temperature regions. The marine forms are basically tropic in distribution, but some individuals stray into temperate waters.